1. Field of the Invention
The present invention pertains to references for calibrating instruments for measuring optical wavelength or frequency, such as optical spectrum analyzers and wavelength meters.
2. Art Background
Devices for measuring the wavelength or frequency of optical signals, such as optical spectrum analyzers or wavelength meters contain a mixture of electronic and mechanical assemblies. Especially with the presence of mechanical assemblies, calibration is an important part of the proper use of these devices. Calibration references provide a set of accurate, known frequencies or wavelengths, either in the form of emission lines, or in the form of absorption lines, that may be used to calibrate instruments.
To be an effective calibration reference, a device must produce these known wavelengths or frequencies with accuracy and stability. One set of references involves using known absorption lines of a gas cell, such as acetylene or methane. When coupled to a wideband light source, such gas cells produce absorption lines which are very stable over environmental changes such as operating temperature, altitude, and humidity. They are also mechanically rugged. Gas cells, while providing physical references, only provide information over a limited frequency or wavelength range. Often times gases to provide references at other wavelengths or frequencies are unavailable, or have undesirable properties, such as degrading over time, or posing handling problems.
Calibration solutions not using gas cells have used expensive technologies which provide calibration not based on physical references. These solutions, such as external cavity lasers, or environmentally compensated etalons, have two main characteristics. First, they are calibrated at the factory during manufacture. Second, they must be designed to have very little change in wavelength over shifts in environmental conditions, or over the trials and travails of transportation, and over time. It is difficult to design such solutions to be stable and yet transportable, and not requiring periodic recalibration against more stable references.
What is needed is an optical calibration reference which is stable, does not require factory recalibration, does not need to be designed to remain constant over environmental change or the effects of time, and provides reference information over a broader band than gas references.
The calibration reference of the present invention combines a first device, such as a gas cell, which provides a physical reference, with a secondary device which provides predictable variation in amplitude vs. wavelength or frequency. The first device is used as a transfer reference with the secondary device to provide calibration information for that secondary device. The first device is used to calibrate the secondary device over the range of the first device. This information and the response characteristics of the secondary device are then extrapolated to other frequencies or wavelengths of interest outside the range of the first device. One embodiment of the present invention uses gas cell as the first device, and an etalon such as a Fabry-Perot (FP) filter for the second device. Another embodiment of the present invention uses a Mach-Zehnder interferometer as the second device. Another embodiment of the present invention combines the etalon and the gas cell into a single device.